Outflows from inflows: the nature of Bondi-like accretion

Abstract

The classic Bondi solution remains a common starting point both for studying black hole growth across cosmic time in cosmological simulations and for smaller scale simulations of AGN feedback. In nature, however, there will be inhomogenous distributions of rotational velocity and density along the outer radius (R o ) marking the sphere of influence of a black hole. While there have been many studies of how the Bondi solution changes with a prescribed angular momentum boundary condition, they have all assumed a constant density at R o . In this Letter, we show that a non-uniform density at R o causes a meridional flow and due to conservation of angular momentum, the Bondi solution qualitatively changes into an inflow-outflow solution. Using physical arguments, we analytically identify the critical logarithmic density gradient |∂ ln ρ/∂θ| above which this change of the solution occurs. For realistic R o , this critical gradient is less than 0.01 and tends to 0 as R o → ∞. We show using numerical simulations that, unlike for solutions with an imposed rotational velocity, the accretion rate for solutions under an inhomogenous density boundary condition remains constant at nearly the Bondi rateṀ B , while the outflow rate can greatly exceedṀ B .

Mechanical feedback effects on primordial black hole accretion

“…(2) with the sound speed of the medium (Bondi 1952). In that situation, accretion would be closer to the spherical case, at least on scales r acc (but not necessarily in the close vicinity of the PBH; Waters et al 2020). This would be the case, for instance, at redshift z 20, but we do not investigate that epoch here due to its high complexity, as it coincides with the formation of the first structures.…”

“…However, it is expected that small perturbations, which would be certainly present in a cosmological context, would lead to a break of the axisymmetry of the accretion process, which can be then enhanced by instability growth. This can lead to the accretion of some angular momentum (see, e.g., Foglizzo et al 2005;Lora-Clavijo et al 2015; see also Waters et al 2020 for the spherical case). In fact, density inhomogeneities accreted with different impact parameters will already provide some angular momentum.…”

The limits of cosmology: role of the Moon

“…However Bondi–Hoyle–Lyttleton accretion rates are highly uncertain especially in the generic limits of inhomogeneous and anisotropic accretion, by some two orders of magnitude once realistic geometries and inhomogeneities [43] are included. Indeed, accretion may even be dominated by mechanical feedback from BH-driven outflows [44,45].…”

The relationship between black hole accretion rate and gas properties at the Bondi radius

“…The black hole growth and accretion mode are determined by the Bondi solution. The Bondi accretion model is simple but has some problems (Waters et al 2019). For example, in the Bondi solution, it is assumed that the accretion rate is constant with radius in the region between black hole horizon and Bondi radius.…”